Abstract: A modular telecommunication system comprises a plurality of switching blocks and a plurality of inter-connecting lines for interconnecting the switching blocks. Each of the switching blocks includes a multiplicity of line terminators each adapted to be connected to a respective data terminal. A communications controller is provided in each switching block for controlling local data exchange between pairs of line terminators connected thereto across real channels. A virtual channel controller is connected to the communications controller and controls remote data exchange across virtual channels to different switching blocks. Interface units are arranged between the virtual channel controller and a respective one of the inter-connecting lines for providing remote data exchange between data terminals each associated with a different switching block.